The invention relates to electrical circuits in general and more particularly to an improved circuit for determining the slope of a signal, particularly a signal which is not free from disturbances.
In various types of devices there is a need to measure the slope of an electrical signal. For example, the operational suitability of radio interference suppression resistors inserted at various points in an ignition system of an internal combustion engine depends on the resistors remaining within a resistance range between an upper and lower limit. If the resistance is too low, the suppression is inadequate and if the resistance is too high the ignition of the engine does not operate optimally. The interference suppression resistors can change their resistance in the course of the operation of an engine for various reasons to the extent that they will fall outside the specified range. In diagnosing the internal combustion engine, it is therefor desirable that the resistance of the interference suppression resistors be checked. Measuring the resistance of the individual resistors would require a large deal of time consuming labor. However, the present invention starts out from the recognition that the rise of what is known as the arc voltage line can be used as a measure of the resistance of the radio interference suppression resistors. What is meant by the arc voltage line is the waveform of the high voltage present at a spark plug after the first breakdown between its electrodes with the ignition spark burning. Thus, a measure of the resistance can be obtained by measuring the slope of the arc voltage line. For this purpose it is sufficient to connect a suitable measuring instrument to a portion of the ignition system carrying the ignition voltage.
However, the arc voltage line is not a steady curve. Instead, over the major portion of the waveform it is full of spontaneously occurring transients which result in part from the processes within the combustion chambers of the cylinders of the engine. However, between these transients there are undisturbed sections of the arc voltage line. These sections are of sufficient duration to give a rough indication of the resistance of the interference suppression resistors. Transients and undisturbed sections are distributed, as noted above, on the arc voltage line in an unpredictable manner. Thus, for this particular application it becomes apparent that there is a need for a device which can measure the slope of a voltage which contains transients hereon. In other words a device which can select from the total waveform an undisturbed portion is necessary.